Treatment of dross

ABSTRACT

Lead constituents may be removed from a dross to produce a copper matte low in lead assay by melting the dross at temperature in excess of 900° C., allowing the molten dross to separate into a lead bullion layer and a matte layer, removing the lead bullion layer from the matte layer, then heating the matte layer to a temperature in excess of 1000° C., and removing the lead sulphide therefrom as a fume.

TECHNICAL FIELD

This invention relates to a method for treatment of lead drosses, andmore particularly to a pyrometallurgical method for removing leadconstituents from the copper constituents of the dross.

The invention is herein described by way of example only with referenceto dross obtained in the smelting of sintered lead concentrates but isnot limited to drosses obtained from that process.

BACKGROUND ART

In the extraction of lead from lead ores by the sinter process, leadsinter is smelted in a blast furnace to produce bullion and slag whichare separated externally to the blast furnace in a forehearth. Leadbullion from the forehearth is collected in cooling pots and thentransferred to drossing kettles. Copper compounds with other impuritiesrejected from solution during cooling float to the surface in thedrossing kettle. Coke, sawdust, and the like may be added to aid theformation of a dry powdery dross which is skimmed from the surface ofthe cooling lead. Sulphur may be added to remove final traces of copperfrom bullion.

Constituents of the skimmed off lead dross include about 20% by weightof lead sulphide, about 20% by weight of copper sulphide predominentlyas chalcocite, and about 50% by weight of entrained metallic lead metal.Minor amounts of antimony, arsenic and other impurities are alsopresent. Typical assays for various types of dross are shown in Table I.It can be seen that it is not unusual for a dross of the type underdiscussion to have a lead assay of the order of 70% and a copper assayof from about 5% to about 35%.

Various methods have been proposed with the objective of recovering leadfrom dross. These include pyrometallurgical methods and hydroextractivemethods. Of the pyrometallurgical methods U.S. Pat. Nos. 2,217,981 and4,033,761 each describe processes in which lead bullion entrained in thedross is released but neither process enables recovery of bound leadpresent in the dross as lead sulphide, nor does either permit the copperto be recovered readily by virtue that the copper matte retains a highresidual lead assay.

U.S. Pat. No. 4,333,763 provides a process which enables the copperconstituents to be recovered with a sufficiently low lead content to besuitable for shipment to a copper refinery. In that process a pool ofmolten lead metal is first established and then metallic alkali metal,for example sodium metal, is incorporated in the lead pool. The leadsulphide bearing dross is added to the molten lead and alkali metal withstirring at a temperature not above 650° C. The alkali metal reacts withthe lead sulphide to reduce the combined lead to metallic lead andsubsequently a matte phase comprising sulphur compounds of the alkalimetal together with copper is separated from the molten lead. The alkalimetal sulphides may be leached from the matte phase. As the processpermits the handling of recovered copper matte in a conventional copperrefining process, the process has considerable economic advantages incomparison with earlier processes. However the requirement for metallicsodium renders the process expensive to conduct, especially at locationsremote from sodium manufacture.

An object of the present invention was the development of apyrometallurgical process which avoids the disadvantages of the processdescribed in U.S. Pat. No. 4,333,763 and which in preferred embodimentswould enable the copper constituents to be recovered sufficiently freefrom lead constituents to be able to be processed in a copper smelterwith little or negligible increase in contamination.

DISCLOSURE OF THE INVENTION

According to one aspect the invention consists in a pyrometallurgicalmethod for removing lead constituents from copper constituents of adross comprising the steps of:

1. melting the dross at a temperature in excess of 900° C.;

2. allowing the molten dross to separate into a layer of lead bullionand a layer of matte;

3. removing the lead bullion layer from the matte layer;

4. heating the matte layer to a temperature in excess of 1000° C. andremoving at least a major portion of the lead sulphide therefrom as afume whereby to produce a copper matte low in lead assay; and

5. recovering the lead as fume.

PREFERRED EMBODIMENTS

Preferred embodiment of the invention will now be described by way ofexample only. Batches of dross having the composition shown in Table IIwere melted by addition to a slag contained in a vertical furnace heatedby means of a submerged combustion lance at a temperature of from 950°C. to 1200° C. In each case a pool of molten lead formed as a layerwhich could be tapped off leaving a remaining molten matte layercomprising lead sulphide and copper sulphide. The composition of matteand metal is shown in Table III. The higher the temperature the morelead sulphide is eliminated and the greater the concentration of copperin matte.

The separated matte, substantially free of entrained bullion, but richin lead sulphide was fed at 120 kg. per hour into a 250 kg. uprightreactor heated by means of a submerged combustion SIROSMELT lance fedwith an excess of oil to air (95% stoichiometry). Lead sulphide wasremoved by fuming, the lance combustion gases assisting removal of fume,and the fume was recovered in a bag house filter.

The residual copper matte had a composition comprising 80.3% copper,17.1% sulphur and assayed 1.95% lead. The method thus produced a copperproduct with a lead to copper ratio which is acceptable for copperprocessing. After allowing for the cost of oil, operating costs werefound to be much lower than the costs of operating a process in whichsodium metal is consumed and other disadvantages of the sodium processare avoided.

The melting step may be performed at a temperature above about 900° C.although a temperature of 1000°-1200° C. is preferred. It is notessential that the liquid lead layer be separated from the matte andslag layers while both are molten. For example the melt could be castand allowed to separate while cooling. The solidified layers may then bephysically separated and the separated matte layer may be remelted forthe fuming step. However it is economically preferable to tap-off thelead layer and to proceed directly from the liquation and separation tothe fuming step.

Use of the SIROSMELT lance greatly assists the fuming step. A slag isused to protect the lance during the fuming for example a copperreverbratory furnace slag containing about 10% additional lime or a highlead oxide slag.

As will be apparent to those skilled in the art from the teaching hereofthe process conditions may be varied to an extent without departing fromthe inventive concept hereof and such variations are deemed to be withinthe scope of this disclosure.

                  TABLE I                                                         ______________________________________                                        TYPICAL DROSS ASSAYS                                                                     Pb %  Cu %    As %    Ag g/t                                                                              S %                                    ______________________________________                                        1.  Unrecycled coke                                                                            65-75    8-11 0.6-0.8                                                                             --    6-9                                    dross fines                                                                   (skimmed).                                                                2.  Coke dross lumps                                                                           40-65   17-35 --    --    10-11                                  (skimmed).                                                                3.  Recycled coke                                                                              50-65   20-35 1     1200                                         dross fines                                                                   (on kettle)                                                               4.  Recycled coke                                                                              65-70   11-15 0.4-0.8                                                                             2100                                         dross (skimmed)                                                           5.  Sulphur dross                                                                              83-85   5-7   --    --    6-7                                ______________________________________                                    

                  TABLE II                                                        ______________________________________                                        COMPOSITION OF DROSS SAMPLES                                                                       Mineralogical                                            Assay % Net          Compositions                                             ______________________________________                                        Pb      68.0         PbS      23.6%                                           Cu      14.3         Cu.sub.2 S                                                                             15.7%                                           S       6.46         Cu.sub.3 As                                                                            2.44%                                           As      0.69         Pb Metal 47.6%                                           ______________________________________                                    

                  TABLE III                                                       ______________________________________                                        % WEIGHT COMPOSITION OF MOLTEN DROSS                                                          Cu      Pb     S                                              ______________________________________                                         950° C.                                                                          Matte Layer                                                                              29.5      54.2 14.5                                                Metal Layer                                                                              1.25                                                    1020° C.                                                                          Matte Layer                                                                              35.6      49.5 15.0                                                Metal Layer                                                                              1.6                                                     1200° C.                                                                          Matte Layer                                                                              55.6      26.6 17.5                                                Metal Layer                                                                              2.0                                                     ______________________________________                                    

I claim:
 1. A pyrometallurgical method for separating lead constituentsincluding lead and lead sulfide from copper constituents includingcopper sulfide of a dross essentially free of alkali metal additives,the process comprising in sequence:melting the dross at a temperature inexcess of 900° C.; waiting for the molten dross to separate into a layerof lead bullion and a layer of matte, said matte containing copper andlead predominantly in the form of lead sulfide; removing the leadbullion layer; heating the matte layer after removal of the lead bullionlayer to a temperature in excess of 1000° C. by means of a combustionlance submerged in said matte causing at least a major portion of saidlead sulfide to leave said matte in the form of lead sulfide fumethereby producing a copper matte sufficiently low in lead assay to besent to a copper recovery operation without further treatment; andrecovering said lead sulfite as lead fume.
 2. The method of claim 1wherein the step of melting the dross is conducted at a temperaturewithin the range between 1000° and 1200° C.
 3. The method of claim 1wherein the step of melting the dross is conducted by means ofcombustion lance submerged in the melting dross.
 4. The method of claim2 wherein the step of melting the dross is conducted by means of acombustion lance submerged in the melting dross.
 5. The method of claim1 wherein said lead bullion layer is removed while in the liquid state.6. The method of claim 2 wherein said lead bullion layer is removedwhile in the liquid state.
 7. The method of claim 1 wherein saidsubmerged combustion lance is fed with an excess of oil fuel to air at95% stoichiometry.